common difference
The common difference is the fixed gap you add to move from one term to the next in an arithmetic sequence — the constant rise of each step on the staircase. It is written d.
You find it by subtracting any term from the one right after it: d = a_2 − a_1 = a_3 − a_2, and so on. A positive d makes the sequence increase, a negative d makes it decrease, and d = 0 gives a sequence that just repeats one value.
The word “common” means the same difference appears between every adjacent pair. If the gaps are not all equal, there is no common difference and the sequence is not arithmetic; in that case you might instead look for a common ratio (multiplication) to see whether it is geometric.
For 20, 17, 14, 11, ..., d = 17 − 20 = −3, a steady decrease of 3 each term.
A negative common difference gives a decreasing sequence.